Determining the energy content of mixtures containing hydrocarbons and other organic compounds can be important in a number of applications. Energy feedstocks, such as natural gas, liquid natural gas, crude and refined petroleum products, and the like may be valued and traded based on the energy content of the mixture. Even small offsets in the energy content (or BTU) value of the mixture can potentially translate to significant energy value differences. For example, in natural gas pipelines, very large quantities of hydrocarbon mixtures having variable composition are transported over very short periods. The energy density of natural gas is typically desirably maintained at or near 1050 BTU/ft3 to ensure proper, energy efficient operation of turbines, furnaces, and other combustion systems and to reduce the potential for damage to equipment. Non-optimized operation can create excessive CO and CO2, due to under efficient energy extraction and can also create costly damage to power generation turbines. Real time or near real-time BTU measurement of the turbine feeding gas stream is essential to prevent equipment damage and minimize emission of CO and CO2.
Energy content measurements for a mixture can be performed with a calorimeter. For this technique, a mixture to be analyzed is fed to a calorimeter chamber at a known, constant rate to a jet at which the reaction occurs. Alternatively, a known volume of the mixture is analyzed in a batch manner. The reaction chamber and gas pipes are contained in a thermostatically controlled water bath to ensure constant temperature. The reaction is then started and the temperature rise measured after a known amount of gas has been fed into the reaction. Calibration of the calorimeter, either with a standard reaction or by electrical means, allows calculation of the enthalpy (ΔH) of the reaction because the reaction is conducted at constant pressure. Flame calorimetry can be performed in near real time depending on the design of the device employed. However, if the heated mass has a large heat capacity, it will take longer to register a meaningful temperature shift which can result in a delay. Ambient heat loads can also introduce potential measurement delays and/or errors. In addition, calorimeter design is very difficult, especially for processes involving very small energy changes, e.g., energy changes on top of a large background such as pipeline gas. Maintenance and calibration of these devices can also require considerable resources.
Analyzers which calculate energy content by measuring residual oxygen left over from combustion of the gas provide indirect measurements that can also be susceptible to various errors. For example, oxygen can be consumed in the formation of nitrogen oxides from N2 present in the mixture being tested, which can lead to over-estimation of the energy content. Furthermore, oxygen probes used in such sensors can be limited in their sensitivity and can have operating temperature limitations that could prevent their use in pyrolysis ovens. Cooling and further handling can also be required, thereby leading to additional analytical errors. Use of paramagnetic oxygen sensors can also limit the operating temperature and resolution. Incomplete oxidation of the mixture can lead to additional errors. The use of an oxygen stream for the oxidation can also lead to increased operating costs.
Sampling of the mixture composition by gas chromatography (GC) can be used to separate and quantify each species that is present. However, gas chromatography can have difficulty speciating compounds with 5 or 6 to 10 or more carbon atoms and can be susceptible to interference by contaminants such as water, sulfur compounds, and other mixture components that can foul the GC column. Additionally, GC measurements can take several minutes or even longer per sample. At pipeline pressures and transport speeds typical for natural gas, this delay can potentially lead to substantial billing errors, on the order of thousands of dollars. Gas chromatography also cannot determine the hydrogen concentration of a gas stream, so the total energy content can potentially be under-represented by such a measurement. Operation and maintenance costs of operating GCs can also be quite large due to the required consumable carrier gases and the regular maintenance required to assure that the instrument will continue to provide accurate data.